西藏泽当地幔橄榄岩经历了碳酸盐熔体交代——单斜辉石的矿物化学证据  

The Zedang mantle peridotites in Tibet metasomatised by carbonate melts: Mineralogical and chemical evidences of clinopyroxenes

作　　者：李勇明 郭国林[1] 刘晓东[1] 张勇[2] 严兆彬[1,2] LI Yong-ming;GUO Guo-lin;LIU Xiao-dong;ZHANG Yong;YAN Zhao-bin(State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,China;College of Earth Science,East China University of Technology,Nanchang 330013,China)

机构地区：[1]东华理工大学核资源与环境国家重点实验室,江西南昌330013 [2]东华理工大学地球科学学院,江西南昌330013

出　　处：《矿物学报》2022年第4期487-502,共16页Acta Mineralogica Sinica

基　　金：国家自然科学基金项目(编号:41762005);核资源与环境国家重点实验室自主基金(编号:2020Z04)。

摘　　要：泽当地幔橄榄岩中单斜辉石的矿物化学成分总体呈现高 CaO、Mg O、Al_(2)O_(3)和低 Cr_(2)O_(3)、Na_(2)O、Ni O 的特征,从二辉橄榄岩、方辉橄榄岩到纯橄岩,单斜辉石的 Mg^(#)值( [100w(Mg)/w(Mg+Fe)],68.0~97.4)和 Cr_(2)O_(3)含量升高,Al_(2)O_(3)含量降低。单斜辉石的元素双变量图解显示,二辉橄榄岩处于低程度部分熔融的深海地幔橄榄岩区域,纯橄岩位于高程度部分熔融的弧前地幔橄榄岩区域,暗示泽当地幔橄榄岩可能经历了从大洋中脊(MORB)到洋内俯冲(SSZ)的两期构造过程。泽当地幔橄榄岩中单斜辉石的 Ca/Al 比值介于 4.80~58.69,其中二辉橄榄岩为 4.80~28.42,方辉橄榄岩为 19.12~44.83,纯橄岩为 25.07~58.69,指示可能经历了碳酸盐熔体的交代改造。矿物微区微量元素分析表明,泽当地幔橄榄岩中的单斜辉石具有高 Zr/Hf 比值(>38,个别高达 200 以上)和低的Ti/Eu 比值(<1500)的特征,与碳酸盐熔体的交代特征相符。因此,我们认为泽当地幔橄榄岩可能早期形成于大洋中脊环境(MORB),后期又受到了俯冲带(SSZ)熔体的交代改造,单斜辉石的矿物主/微量指示该熔体类型可能为碳酸盐熔体。The chemical compositions of clinopyroxenes in the Zedang mantle peridotites are generally characterized with high contents of CaO, Mg O, and Al_(2)O_(3), low contents of Cr_(2)O_(3), Na_(2)O and Ni O. The Mg# values(68.0-97.4) and the Cr_(2)O_(3)contents are increased while the Al_(2)O_(3)contents are decreased for clinopyroxenes respectively selected from lherzolite, harzburgite, and dunite. The bivariate geochemical diagrams of elements in clinopyroxenes show that clinopyroxenes of the lherzolite are plotted in the field of the deep-sea mantle peridotite, while clinopyroxenes of the dunite are plotted in the filed of fore-arc mantle peridotite, which indicate that the Zedang mantle peridotite could have been experienced two stages of tectonic processes of the mid-ocean ridge(MORB) and the supra-subduction zone(SSZ). The Ca/Al ratios of clinopyroxenes in the Zedang mantle peridotites vary from 4.80 to 58.69. Especially, those of clinopyroxenes in the lherzolite vary from 4.80 to 28.42, those of clinopyroxenes in the harzburgite vary from 19.12 to 44.83, and those of clinopyroxenes in the dunite vary from 25.07 to 58.69. It is indicated that these mantle peridotites could have been metasomatised by carbonate melts. In situ micro analyses of trace elements in minerals show that clinopyroxenes in the Zedang mantle peridotites are characterized with high Zr/Hf ratios( > 38, occasionally higher than 200) and low Ti/Eu ratios(< 1500), which are consistent with characteristics of clinopyroxenes metasomatised by a carbonate-type melt. Therefore, it is believed that the Zedang mantle peridotites were formed in the mid-ocean ridge(MORB) environment in the early stage and were metasomatised by melt in the supra-subduction zone(SSZ) in the late stage. The compositions of major/trace elements of clinopyroxenes indicate that the melt caused the metasomatism of the Zedang mantle peridotites could be carbonate-type melt.

关 键 词：西藏泽当 蛇绿岩 地幔橄榄岩 单斜辉石 部分熔融 碳酸盐熔体交代 

分 类 号：P588.125[天文地球—岩石学]